The invention relates to turbine airfoil construction and, more particularly, to a turbulator configuration in the concave interior surface of an airfoil leading edge.
In general, increased internal cooling magnitudes are desired for any cooled gas turbine airfoil. The leading edge cooling passage of any such airfoil experiences the highest heat load on the airfoil, and so requires the highest degree of internal cooling. This requirement is much more highly evident for closed-circuit cooled airfoils, such as the steam-cooled buckets of General Electric's H-system turbine® (but the requirement holds for all cooled turbines). Solutions that allow high heat transfer coefficients, uniformity of heat transfer, and also lower friction coefficients are continuously sought. Any solution should also be manufacturable, preferably by investment casting methods.
In open-circuit air-cooled turbine airfoils, solutions generally include the increase of film cooling in the airfoil leading edge to compensate for lower internal heat transfer, or the increase in impingement heat transfer into the concave leading edge passage if enough pressure head is available. Swirl cooling by wall-jet injection is another solution. In closed-circuit cooled airfoils, solutions generally revolve around limited forms of turbulation on the concave surface.
The primary solution in the current art for closed-circuit cooling is the use of transverse repeated turbulators, i.e., the turbulators are arranged substantially perpendicular to a longitudinal axis of the passage. FIG. 1 shows the prior art layout of a concave cooling passage 2 including transverse turbulators 3. FIG. 2 is an end view showing the concave shape of the cooling passage. If the turbulators 3 are transverse and each a continuous strip, they act in the conventional manner by tripping the flow to provide mixing. The conventional methodology leads to high heat transfer and high friction coefficients. This is the case regardless of the concave shape of the airfoil leading edge.
It has been proposed to angle the turbulators 3 to the flow as shown in FIG. 3. If the turbulators 3 are angled to the flow, such as the 45° angled version of FIG. 3, but still of continuous form within the concave portion, then a portion of the flow is diverted to follow the turbulators 3 near the surface creating a swirling flow in the semi-circular shaped passage 2. This serves to substantially lower the coefficient of friction while also delivering a high heat transfer coefficient. The uniformity of the heat transfer however is not high. Also, this geometry is not amenable to an investment casting process because the turbulators 3 are continuously angled across the concave surface. The variation in cast shape of these turbulators 3 will be large, with regions of undesirable turbulator lean or size.
It would thus be desirable to provide a leading edge construction with a turbulator arrangement that effects high heat transfer with lower friction losses while also being castable by investment casting methods.